Among automotive parts, a rack bar, which is used for a steering apparatus, is an important part which steers the travelling direction of the automobile and also serves as a framework to link the left and right wheels so that if it fails, steering operation will be disabled. For this reason, high reliability is required for steel materials to be used as a rack bar.
A rack bar has been produced by subjecting a rolled round steel material of middle carbon steel to thermal refining treatment by quenching and tempering, then to drawing as needed, and further to boring and gear tooth shaping (forming of a tooth shaped part) by machining, and subjecting the tooth shaped part to induction quenching and tempering.
For that reason, it is required for a rack bar that even when subjected to an excessive load, a crack generated in an induction hardened layer should not propagate in the base metal, leading to rupture.
Further, a rack bar is subjected to a deep hole machining in the longitudinal direction of a radially central part.
Therefore, to achieve productivity and characteristics of component at the same time, a round steel material to be used as a starting material for a rack bar is required of excellent machinability when machining a deep hole, and excellent base metal impact characteristics (base metal toughness) for resisting the propagation of a generated crack.
As the steel material to be used for such a rack bar, the present inventors have proposed, for example, the following steel materials.
They proposed, in Patent Document 1, a rolled steel material for induction quenching having: a chemical composition consisting of, in mass %, C: 0.38 to 0.55%, Si: not more than 1.0%, Mn: 0.20 to 2.0%, P: not more than 0.020%, S: not more than 0.10%, Cr: 0.10 to 2.0%, Al: not more than 0.10%, and N: 0.004 to 0.03%, with the balance being Fe and impurities, wherein the value of fn1 which is represented by a Formula [fn1=C+( 1/10)Si+(⅕)Mn+( 5/22)Cr+1.65V−( 5/7)S](where C, Si, Mn, Cr, V and S respectively represents content in mass % of each element) is not more than 1.20; and a microstructure consisting of ferrite, lamellar pearlite, and spheroidal cementite, wherein an average grain diameter of ferrite is not more than 10 μm, an area proportion of the microstructure occupied by lamellar pearlite having a lamellar spacing of not more than 200 nm of the entire lamellar pearlite is 20 to 50%, and a number of particles of spheroidal cementite is not less than 4×105/mm2.
It is noted that the rolled steel material for induction quenching may further contain one or more elements selected from Cu, Ni, Mo, Ti, Nb, and V.
They proposed, in Patent Document 2, a rolled steel material for induction quenching having: a chemical composition consisting of, in mass %, C: 0.38 to 0.55%, Si: not more than 1.0%, Mn: 0.20 to 2.0%, P: not more than 0.020%, S: not more than 0.10%, Cr: 0.10 to 2.0%, Al: 0.010 to 0.10%, and N: 0.004 to 0.03%, with the balance being Fe and impurities, wherein the value of Ceq which is represented by a Formula [Ceq=C+( 1/10)Si+(⅕)Mn+( 5/22)Cr+1.65V−( 5/7)S](where C, Si, Mn, Cr, V and S respectively represents content in mass % of each element) is not more than 1.20, and a total content of Si, Mn, and Cr is 1.2 to 3.5%; and a microstructure consisting of ferrite, lamellar pearlite, and spheroidal cementite, wherein an average grain diameter of the ferrite is not more than 10 μm, an area proportion of the microstructure occupied by the lamellar pearlite is not more than 20% (including 0%), and a number of particles of spheroidal cementite is not less than 6×105/mm2.
It is noted that the rolled steel material for induction quenching may further contain one or more elements selected from Cu, Ni, Mo, Ti, Nb, and V.